In many communications systems, for example, wireless communications systems such as mobile satellite radiotelephone systems, it is generally desirable to use as little signal power as necessary to communicate signals. Toward this end, many communications systems utilize error correction coding to improve signal-to-noise ratio without requiring an increase in signal power. As illustrated in FIG. 1, in a typical wireless communications system, information represented by information symbols 105 is first encoded according to an error control code, for example, in a convolutional encoder 110. The coded symbols 115 output from the convolutional coder 110 are then typically interleaved in an interleaver 120, which reorders the coded symbols 115 to produce a plurality of interleaved coded symbols 125. The interleaved coded symbols 125 are then typically orthogonally modulated by an orthogonal modulator 130 to produce a communications signal 135 which is then communicated over a communications medium 140. An orthogonal modulator 130 is typically employed because the orthogonally modulated communications signal 135 can be demodulated in a noncoherent fashion at a receiver.
The convolutional encoder 110 and the orthogonal demodulator 130 produce an overall code rate and minimum distance for the communications signal 135. The convolutional encoder 110 typically employs a rate k/n convolutional code, i.e., a code which produces n coded symbols 115 for each k information symbols 105 input into the encoder 110, and generally has a minimum Hamming distance d.sub.1,min associated therewith. The orthogonal modulator 130 typically employs a set of p orthogonal block modulation vectors of length l for input vectors of length m, producing a code rate of m/l and a minimum distance of d.sub.2,min. When combined as illustrated in FIG. 1, the convolutional encoder 110 and the orthogonal modulator 130 produce an overall code rate of k/n * m/l and a minimum distance of d.sub.1,min * d.sub.2,min.
Although techniques exist for improving the error-correcting performance of a conventional combination of a convolutional encoder and an orthogonal modulator, for example, lowering the rate of the convolutional encoder 110 or the orthogonal modulator 130 to provide greater redundancy, these techniques can result in lower efficiency and greater complexity in receiver design. Due to bandwidth and power constraints, there is an ever present need to provide improved signal-to-noise performance without requiring undue additional complexity in receiver design, increased signal power and decreased rate.